A recent development in the field of electro-mechanics has been the miniaturization of various electro-mechanical devices, called "micro-mechanical" devices. These micro-mechanical devices are typically manufactured using MOS and other integrated circuit techniques. Micro-mechanical devices include tiny motors, microgears, and a type of spatial light modulator CSLM") comprised of items often referred to as "DMD,s" which stands for "deformable mirror device" or, more recently, "digital micromirror device."
A DMD SLM includes one or more electrically conductive, selectively movable reflectors or mirrors. Light incident on a DMD is selectively reflected or not reflected to a reception site, depending on the position of the mirrors.
Each mirror is carried by one or more deformable beams. The beams may be cantilever, torsion or flexure (cantilever+torsion) beams. The beams are supported by posts or spacers so that each mirror is maintained over and spaced from a substrate. The distance between each mirror and the substrate permits the mirror to move relatively to the substrate--toward and/or away therefrom--out of a normal position in which no energy is stored in its beams. Movement of a mirror deforms and stores energy in its beams which tends to return the mirror to its normal position.
Each mirror is selectively moved by control circuitry monolithically formed in and on the substrate. The circuitry may include one or more selectively energizable control electrodes and one or more landing electrodes on the substrate. Application of appropriate voltages to selected control electrodes and to selected mirrors (via their beams) produces an electrostatic field therebetween which effects movement of the selected mirrors away from or, as is more typical, toward the substrate. Mirror movement may be analog--proportional to the strength of the electrostatic field'bistable or tristable. In the latter two events, the electric field is simply rendered sufficiently strong to move the mirrors until they abut or contact a "stop."
Analysis of non-analog operation of DMD's indicates that neither the substrate nor the control electrode is a desirable stop for its associated mirror. Engagement between the mirror and either of these items typically leads to the mirror becoming "welded" thereto or destruction of the mirror. As a consequence, the landing electrodes serve as stops, that is, they are contacted or engaged by the edges of the mirrors. The landing electrodes are rendered electrically neutral relative to the mirrors, that is, they and the mirrors are at the same potential. Accordingly, engagement of the landing electrodes by the mirrors avoids welding or destruction of the mirrors.
However, reliability has been difficult to achieve with micro-mechanical devices. A common reliability problem is adhesion or sticking, which occurs when relatively movable elements contact each other. If these elements stick or adhere together, the device may cease to operate properly. A related problem is that repeated contact between elements or micro-mechanical devices can cause their surfaces to wear.
Sticking or adhesion in DMD's has been theorized to be caused, inter alia, by intermetallic bonding between mirrors and their landing electrodes and/or by the high surface energy of the contacting members due to the material thereof or to contaminants sorbed by or otherwise resident on their surfaces. Whatever the cause of sticking or adhesion, which such occurs, the energy stored in a mirror's beam(s) following its movement out of the normal position may be insufficient to return the mirror to its normal position.
To avoid the foregoing problems associated with contacting or engaging elements, many micro-mechanical devices are designed so as not to include elements that come into contact with each other. However, such design restrictions may unduly limit the versatility of micro-mechanical devices and, in any event, would, if applied to non-analog DMD's, absolutely prevent the operation thereof.
Commonly assigned U.S. patent application, Ser. No. 08/216,194, filed Mar. 21, 1994, now U.S. Pat. No. 5,447,600, patented Sep. 5, 1995, describes one approach to avoiding sticking or adhesion of contacting elements of micro-mechanical devices, including the mirrors and landing electrodes of DMD's. Specifically, that application describes the application, during DMD fabrication, of a solid polymeric layer to that portion of one of the contacting elements (e.g., the landing electrode) which contacts the other element (e.g., the mirror). Preferred polymers include fluoropolymers, such as Teflon-AF. Other approaches are set forth in commonly assigned U.S. application Ser. No. 08/220,429, filed Mar. 30, 1994 TI-18470!.